Stopping and starting a vehicle by manual operation of the accelerator and brake pedals involves subtle handling of these pedals (and also of the clutch pedal), and requires the brake pedal to be depressed constantly while the vehicle is at a standstill. It thus demands a fair degree of experience on the part of the operator, while the onus on him is great. This is particularly true when stopping and starting on a hill.
For this reason there has long been a demand to facilitate the stopping and starting of a vehicle by means of a simple operation which places as little onus as possible on the operator.
Japanese Patent Application Laid-Open No. 59-143746 proposes holding or releasing braking force by means of controlling an electrically operated parking brake device.
In other words, according to the abovementioned invention, if with vehicular speed at zero the accelerator pedal is released and the brake pedal depressed, the parking brake device operates and stops the vehicle automatically.
This means that if, for instance, an attempt was made to exert subtle control over the vehicle by operating only the accelerator pedal with a view to maintaining a safe distance from a vehicle in front on a steep upward slope, it might in the end become necessary to depress the brake pedal, and the vehicle would sometimes begin to move backwards before this operation was effected.
Moreover, in the abovementioned invention, the engine rotational speed at which it is possible to obtain the output necessary to ensure that the vehicle does not roll backwards on a slope is set in accordance with the gradient of the slope, so that the parking brake is released and the vehicle allowed to move forward once the engine attains a speed corresponding to the gradient of the slope as detected by a gradient sensor.
It is true that in vehicles which employ torque converters the tractive force is more or less determined by the engine rotational speed and the speed stage, and that releasing the brake when the engine has attained a speed corresponding to the current gradient does not create any problem because the necessary tractive force can be obtained. However, in vehicles which employ direct transmission the tractive force is determined by the transmission torque of the main clutch, and not by the engine rotational speed. Moreover, much the same applies also to vehicles which employ other types of transmission and to vehicles driven by electricity, where again the tractive force is not determined on the basis of the engine rotational speed.
Accordingly, in vehicles of this sort it does not follow that the required tractive force can always be obtained even if the engine has attained a speed corresponding to the current gradient when the brake is released.
For this reason there has been a tendency for the tractive force on brake release either to be insufficient, so that the vehicle begins to roll backwards, or to be too great, with the result that the tires slip.
Japanese Patent Application Laid-Open No. 61-200054 similarly proposes a technique for holding or releasing braking force by means of controlling the hydraulic brakes.
That is to say, according to the abovementioned invention, when the gradient (inclined angle) of the vehicle is at a given standard value or more, the foot-brake is applied and the vehicle speed is zero, a cylinder pressure becomes greater than a brake-holding pressure corresponding to the gradient. When the hydraulic pressure falls again and matches the holding pressure, a control valve closes, brake pressure is held, and the brake operates.
In this manner, the fact that the abovementioned invention presupposes that "applying the foot-brake" is an automatic braking action means that as in the case of Japanese Patent Application Laid-Open No. 59-143746 there has been a tendency for the vehicle to begin rolling backwards before the brake pedal is finally depressed.
Moreover, in the abovementioned invention the control valve and brake are released when the degree of opening of the accelerator is greater than a given standard value and the clutch switch is on. Thus, inasmuch as it does not assess whether or not the tractive force corresponds to the current gradient of the road, there has been a tendency on steep gradients for vehicles to begin rolling backwards as soon as the brake is released.